1. Field of the Invention
This invention relates to gas separation systems using molecular sieves and is especially concerned with the employment of such systems in the aviation field for obtaining oxygen-enriched air as a breathable gas for aircrew.
2. Description of the Prior Art
A common manner of operating a molecular sieve-type gas separation system employing one or more molecular sieve beds, involves charging the or each bed with feed gas mixture--e.g. air--under pressure and continuing the feed to the bed to maintain the charge pressure during delivery of the required product gas constituent(s) to use or to storage. When the bed approaches saturation with adsorbed constituents of the feed gas, the bed is regenerated: for this purpose the feed is discontinued and the bed vented to release the charge pressure, whereafter the bed is purged. The pressurising of the sieve bed promotes adsorption of the constituents to be retained by the bed, while the subsequent depressurising promotes desorption of the retained constituents to facilitate flushing of these from the bed.
In aircraft applications it is normally required that the oxygen concentration in the breathable gas supplied to aircrew shall be so related to cabin altitude, i.e. to the ambient pressure obtaining within the aircrew enclosure, that the oxygen partial pressure in the breathable gas is kept within a physiologically acceptable range of values. In this regard, the normally accepted minimum oxygen content of the breathable gas is that required to provide, at all cabin altitudes, the same oxygen partial pressure as at sea level. However in the case of military aircraft intended to operate at very high altitudes, this minimum oxygen content is inappropriate for certain flight regimes. Thus at high altitude provision must be made for the possibilities of rapid cabin depressurisation arising, for instance, from structural damage. In such an event there is a rapid decompression of the breathable gas within the lungs of an aircrew member and it is generally accepted that if loss of consciousness in these circumstances is to be avoided, the oxygen content of the gas in the lungs at the onset of cabin depressurisation needs to be such that it provides a minimum partial pressure of 30 mm Hg at the termination of decompression: that is, when the total gas pressure in the lungs corresponds to atmospheric pressure at the operating altitude. Consciousness can then be maintained if the breathable gas available thereafter has an oxygen content of 100%. For these reasons, at high altitudes the breathable gas supplied to the crew of military aircraft should have an oxygen content giving a partial pressure greater than the sea level equivalent.
On the other hand, under high acceleration forces, parts of the lungs can distort to entrap pockets of gas. If the entrapped gas has a composition such that it can be wholly adsorbed while the entrapment persists, the regions in which it is entrapped can collapse, causing pain and discomfort. This risk of total adsorption of entrapped gas increases with increasing oxygen content--i.e. decreasing inert gas (nitrogen) content--so that it is undesirable, especially in a highly manoeuvrable aircraft that may be subject to high G-forces, to supply a breathable gas of excessive oxygen content. In general the highest acceleration forces mainly occur in manoeuvres at low altitude where the need, for other reasons, for a high oxygen content in the breathable gas does not exist. Accordingly while there are reasons for providing a higher than physiologically necessary oxygen content in the breathable gas at high altitude, there are distinct disadvantages in supplying a breathable gas with a higher than necessary oxygen content at lower altitudes.
These considerations in effect establish at every operating altitude an individual range of oxygen content for the breathable gas to be supplied to the aircrew of a modern high performance military aircraft.
Aircraft on-board oxygen generation systems (OBOGS) based on molecular sieve gas separation technology and operating in the manner outlined above can be made to deliver a product gas with an oxygen content that increases with altitude, by the simple expedient of venting the or each sieve bed, during its regeneration phase, to the external atmosphere (or to the cabin, which has a pressure related to that of the external atmosphere) so that with increasing altitude the bed pressure during desorption reduces, thereby progressively to enhance desorption of retained constituents with increasing altitude. See, for instance, EP-A-0 080 300. However, the "self-regulation" possible by this expedient is limited and to provide better regulation and a closer approach to ideal product gas constitution over an extended altitude range, supplementary control expedients are needed. In the system EP-A-0 080 300 for instance, there is a fixed logic sequencer controlling the sequential operation of charge and vent valves for cyclically subjecting each sieve bed to a charge/adsorption on-stream phase followed by purge/desorption regeneration phase. The control means provide that for a predetermined range of ambient atmospheric pressure (altitude range) the overall cycle time and the relative durations of the phases are fixed at values such that the oxygen content of the delivered product gas remains within physiologically acceptable limits for breathing. The cycle time is modified at one or more predetermined altitude thresholds to provide a suitably extended operating altitude range within which the product gas constitution is acceptable. However because the control means take no account of demand flow rate (which affects the performance of the molecular sieve system) there is a tendency for this OBOGS to deliver over-high oxygen concentration under certain demand flow conditions within certain altitude ranges. This is not only undesirable for the reasons already discussed, but also because the production of excessively oxygen-rich breathable gas represents an excessive use of feed air, usually engine bleed air, for which there are usually competing demands.
EP-A-0 129 304 discloses a molecular sieve-type gas separation system that aims to maintain the simplicity of control provided by the system of EP-A-0 080 300 while providing enhanced control of the product gas constitution under an extended range of demand flow rates. To this end, the system of EP-A-0 129 304 is characterised by means responsive to the pertaining concentration of a desired constituent (e.g. oxygen) in the product gas and arranged to adjust the overall cycle time in such manner that this concentration is maintained within predetermined limits. The control means of the system in EP-A-0 129 304 may comprise a fixed logic sequencer controlling the sequential operation of charge and vent valves as in the system of EP-A-0 080 300. However instead of adjusting this sequencer to vary the cycle time merely by reference to altitude, in the system of EP-A-0 129 304 adjustment of the fixed logic sequencer to vary the cycle time is accomplished by means responsive to the product gas composition, e.g. a transducer sensitive to the oxygen partial pressure of the product gas, thereby to take account both of altitude and demand flow rate and, indeed, other operating parameters that affect the product gas composition.
EP-A-0 129 304 discloses the possibilities of using, as a partial pressure transducer, a galvanic type gas sensor or a flueric partial pressure sensor or a flueric partial pressure sensor (such as disclosed in EP-A-0 036 285). In the system particularly described, the fixed logic sequencer unit provides two different overall cycle time modes and is switched between these in response to signals output by the partial pressure transducer. However it is disclosed that the switch that accomplishes this changeover may be substituted by a variable resistor means to provide a varying voltage output signal for a suitably responsive timer to vary the cycle time steplessly or in a number of steps between predetermined minimum and maximum cycle times, in a manner appropriate to retaining the desired partial pressure of oxygen in the product gas by producing a cycle time adjustment that takes account of the magnitude of sample gas oxygen partial pressure departure from the required value.
The system of EP-A-0 129 304, like the system of EP-A-0 080 300, aims to provide a breathable gas product exhibiting a rising oxygen concentration with cabin altitude increase such as to provide a substantially constant oxygen partial pressure at all cabin altitudes within the operating range, this partial pressure being chosen so as to fall within the permissible range of values for all cabin altitudes. The chosen value is necessarily a compromise and in practice falls close to one or the other limiting values of the permissible range at certain cabin altitudes and, especially, is higher than desirable at low altitudes.